The present invention relates to a method for production of foamed plastics. More precisely, the present invention relates to a method for production of foamed plastics comprising adding particles containing 30 to 70% by weight of water in thermoplastic resin, mixing, heating and melting said thermoplastic resin in which said particles have been added at high pressure, then releasing said high pressure affecting on the resulting mixture to foam said mixture of said thermoplastic resin and said particles, and cooling said foamed mixture.
Up to now, the chemical foaming method wherein a chemical blowing agent which generate gas when decomposed or a solvent having a low boiling point is mixed in a synthetic resin to make expand said synthetic resin foam by heating or the mechanical foaming method wherein air is mixed in synthetic resin by stirring have been provided. Further, the capsule method wherein a blowing microcapsule in which a solvent having a low boiling point is encapsulated is mixed in synthetic resin and heated and foamed has been also provided.
In said chemical foaming method or said mechanical foaming method, it is difficult to make uniform foamed structure and further in the chemical foaming method, the chemical blowing agent is expensive, and in the capsule method, the microcapsule is very expensive.
Accordingly, an object of the present invention is to provide foamed plastics having uniform foamed structure.
Another object of the present invention is to provide a method wherein said foamed plastics having uniform foamed structure can be produced economically.
Said object can be attained by a method for production of foamed plastics comprising adding particles containing 30 to 70% by weight of water in thermoplastic resin, mixing, heating and melting said thermoplastic resin in which said particles have been added at high pressure, then releasing said high pressure affecting on the resulting mixture to foam said mixture of said thermoplastic resin and said particles, and cooling said foamed mixture.
It is preferable that the average diameter of said particles containing the water is 30 to 200 xcexcm and 0.1 to 5 parts by weight of said particles are added in 100 parts by weight of said thermoplastic resin and said particle containing the water is a porous particle of a synthetic resin. Said porous particle of synthetic resin is preferably produced by dispersing the water with a surface active agent in an oil phase which is a mixture of an unsaturated polyester and a vinyl monomer which can react with said unsaturated polyester to form cross linking to prepare an emulsion, and re-dispersing said emulsion in the water and curing said mixture to form cross linking.
Further said particle containing the water is also a porous inorganic particle.
It is also preferable that said thermoplastic resin and said particles containing the water are heated, melted and mixed in a cylinder of a melting mixer, and said high pressure affecting said melted mixture is released by extruding said melted mixture from the nozzle or the die which is attached at the tip of said melting mixer to foam and cool said melted mixture and said melting mixer is preferably that of an injection molding machine and said melting mixer""s nozzle is connected to a mold, and said melted mixture is injected into said mold to be released from said high pressure and to be cooled and said high pressure is preferably set at 5 MPa and over and further releasing of said high pressure affecting said melted mixture is carried out at or below the pressure of the atmosphere is preferably.
In the present invention, it is easy to mix said particles containing water uniformly in the melting thermoplastic resin. In particular, uniform mixing of said particles in said melting thermoplastic resin is very easily carried out when said melting thermoplastic resin is mixed with said particles by the screw in the cylinder of said melting mixer
When the high pressure affecting said melting thermoplastic resin in which said particles are mixed is released to put said melting thermoplastic resin preferably under the pressure of the atmosphere or below, the water contained in said particles evaporate quickly to form the foamed structure in said thermoplastic resin and said foamed structure is fixed by cooling. In the case where the water content of said particle is below 30% by weight, it is hard to form the foamed structure in said thermoplastic resin and in the case where the water content of said particle is beyond 70% by weight, the foamed structure becomes so coarse that said foamed structure may collapse. In the case where the pressure is set to be higher than 5 MPa, a large difference of the pressure is ensured when said pressure is released so that the smooth foaming by the sudden evaporation of the water is preformed.